Calcium carbonate
Calcium carbonate is the most widely used filler in coatings: cheap, bright, easy to disperse and useful in everything from flat wall paints to putties and sealants.
Calcium carbonate (molecular weight 100.09 g/mol) is a white mineral with a density of about 2.7 g/cm³, a Mohs hardness of about 3 and a refractive index of about 1.6. Coatings use ground calcium carbonate (GCC), milled from limestone, marble or chalk, and precipitated calcium carbonate (PCC), a synthetic grade with finer, controlled particle shapes.
It is an extender in architectural paints, especially flat and matt finishes, and in primers, textured coatings, putties, fillers, sealants and plastisols. It reduces cost, adds body, and helps control sheen and opacity.
Calcium carbonate has low toxicity and is generally not classified as hazardous. Control dust when handling powders and follow the SDS and occupational dust limits.
GCC is ground natural rock: cheaper, with a broader particle size range. PCC is precipitated synthetically, giving finer, more uniform particles that can improve brightness and help space titanium dioxide.
Not where acids are involved. It reacts with acids and dissolves, so barium sulfate or silica-based fillers are used instead.
On its own it adds little hiding, because its refractive index is close to the binder’s. Above the critical PVC, the air voids it creates scatter light and increase dry hiding in flat paints.
